1. Field of the Invention
The present invention relates to a cathode ray tube (CRT), and, more particularly, to a CRT having an electron gun in which a cathode for emitting electron beams, a control electrode for controlling emission of the electron beams from the cathode, and a screen electrode for accelerating the flow of the electron beams passing the control electrode are arranged in series.
2. Description of the Related Art
Referring to FIG. 1, a conventional CRT includes a panel 12, a funnel 13, an electron gun 11, and a deflection yoke 15. A fluorescent film 14 in which fluorescent substances for producing red (R), green (G), and blue (B) light are aligned in a dot or strip pattern is installed on the inner surface of the panel 12. The funnel 13 having a neck portion 13a and a cone portion 13b is sealed to the panel 12. The electron gun 11 is installed in the neck portion 13a of the funnel 13. The deflection yoke 15 is installed on and surrounding the cone portion 13b of the funnel 13 for deflecting the electron beams emitted from the electron gun 11.
The performance of the CRT 1 is determined according to a state of the electron beams emitted from the electron gun 11 and landing on the fluorescent film 14. To make the electron beams emitted from the electron gun 11 accurately land on the fluorescent film 14, a number of technologies improving focus characteristics and reducing aberration of electron lenses have been developed.
In particular, the shapes of the electron beams landing on the fluorescent film 14 are horizontally elongated when the electron beams emitted from the electron gun 11 are deflected by the deflection yoke 15, due to a difference between barrel and pincushion magnetic fields. To prevent the elongation, a dynamic focus electron gun is used. The dynamic focus electron gun synchronizes the electron beams emitted from the electron gun 11 with horizontal and vertical deflection periods so that the shapes of the electron beams are vertically elongated.
However, in the dynamic focus electron gun, as the size of the screen of the CRT increases, horizontal line deformation at the peripheral portion of the screen becomes severe. To solve that problem, a double focus CRT is used.
FIG. 2 shows a conventional double dynamic focus CRT. Referring to the drawing, a video signal processing portion 21 processes a composite video signal Sc and outputs a horizontal synchronizing signal, a vertical synchronizing signal, a data signal, and a horizontal/vertical blanking signal. The data signal including red (R), green (G), and blue (B) brightness signals, is amplified by a data signal amplifier 27. The amplified data signal Sd is biased by a voltage supplied by a first bias supplier 31 and applied to a cathode K of the electron gun 11.
A vertical deflecting signal generator 22 generates a vertical deflecting signal corresponding to the vertical synchronizing signal output from the video signal processor 21 and supplies the vertical deflecting signal to a vertical deflecting signal amplifier 24. A horizontal deflecting signal generator 23 generates a horizontal deflecting signal corresponding to the horizontal synchronizing signal output from the video signal processor 21 and supplies the generated horizontal deflecting signal to a horizontal deflecting signal amplifier 25. The vertical and horizontal deflecting signals amplified by the vertical and horizontal deflecting signal amplifiers 24 and 25 are respectively applied to vertical and horizontal deflecting yokes 15 on the CRT 1.
The horizontal/vertical blanking signal output from the video signal processor 21 is amplified by a blanking signal amplifier 26. A horizontal/vertical blanking signal Sb output from the blanking signal amplifier 26 is applied to the cathode K of the electron gun 11. A control signal Vc from a fifth bias supplier 37 is supplied to a control electrode C of the electron gun 11. A heater power supplier 36 supplies electric power to a heater (not shown) of the cathode K of the electron gun 11. A second bias supplier 32 applies a screen voltage Vec to a screen electrode S and a second focus electrode F2 of the electron gun 11. A third bias supplier 33 applies a static focus voltage Vfs having a positive polarity to first, third, and fifth focus electrodes F1, F3, and F5 of the electron gun 11. The static focus voltage Vfs has a positive polarity and a magnitude higher than the screen voltage Vec, which also has a positive polarity, to enhance acceleration and focus of the electron beams. A dynamic focus driver 35 applies a dynamic focus voltage Vfd, which changes periodically within a range above and below the static focus voltage Vfs, to fourth and sixth focus electrodes F4 and F6 so that the electron beams emitted from the electron gun 11 are made relatively oval. A fourth bias driver 34 applies an anode voltage Veb having the highest positive polarity to a final acceleration electrode A of the electron gun 11.
FIG. 3 shows the structure of the electron gun in the CRT of FIG. 2. In FIG. 3, the same reference numerals denote the same elements shown FIG. 2. In FIG. 3, reference characters KR, KG, and KB denote respective cathodes for producing electron beams that generate red, green, and blue light when the electron beams land on the fluorescent screen. Reference character SdR/SbR denotes data and blanking signals for red light, reference character SdG/SbG denotes data and blanking signals for green light, and reference character SdB/SbB denotes data and blanking signals for blue light respectively applied to cathodes KR, KG, and KB.
FIG. 4 shows the relationship between driving voltages in a conventional double dynamic focus method. In FIG. 4, reference character THS denotes horizontal scanning period, reference character Vpl denotes the minimum voltage of the dynamic focus voltage Vfd, and reference character Vph denotes the maximum voltage of the dynamic focus voltage Vfd.
FIG. 5A shows electron lenses formed in the electron gun of FIG. 3 during the period t1-t3, when the static focus voltage Vfs is higher than the dynamic focus voltage Vfd. FIG. 5B shows electron lenses formed in the electron gun of FIG. 3 during the periods 0-t1 and t3-t4, when the static focus voltage Vfs is lower than the dynamic focus voltage Vfd. In FIGS. 5A and 5B, reference character AV denotes the vertical direction in the electron gun, reference character AH denotes the horizontal direction in the electron gun, reference character PB denotes direction of movement of the electron beams, reference character FV denotes the vector force in the vertical direction AV applied to the electron beams, and FH denotes the vector force in the horizontal direction AH applied to the electron beams.
Referring to FIGS. 3, 4, 5A, and 5B, electron beams are generated according to the data signals SdR, SdG, and SdB corresponding to the respective cathodes KR, KG, and KB. The electron beams are emitted in response to the control voltage Vc applied to the control electrode C. The electron beams emitted through openings of the control electrode C are accelerated by the screen voltage Vec applied to the screen electrode S.
The static focus voltage Vfs applied to the first focus electrode F1 is higher than the screen voltage Vec applied to the screen electrode S. The shapes of an outlet of the screen electrode S and an inlet of the first focus F1 are circular, but the outlet of the screen electrode S is smaller than the inlet of the first focus F1. Thus, a focus lens is formed between the screen electrode S and the first focus electrode F1. The shapes of the inlets of the first focus electrode F1 to which the static focus voltage Vfs is applied, the inlets and outlets of the second focus electrode F2 to which the screen voltage Vec is applied, and the inlets of the third focus electrode F3 to which the static focus voltage Vfs is applied are all circular. Therefore, a focus lens SL is formed as a pre-focus lens (SL of FIG. 5A or 5B) among the first, second, and third focus electrodes F1, F2, and F3. The electron beams emitted from the third focus electrode F3 are focused by the focus lens SL.
The shapes of the outlets of the third focus electrode F3 are horizontally elongated while the shapes of the inlets of the fourth focus electrode F4 are vertically elongated. The shapes of the outlets of the fifth focus electrode F5 are vertically elongated while the shapes of the inlets of the sixth focus electrode F6 are circular. The static focus voltage Vfs is applied to the third and fifth focus electrodes F3 and F5 while the dynamic focus voltage Vfd is applied to the fourth and sixth focus electrodes F4 and F6. The anode voltage Veb is applied to the final acceleration electrode A.
In the periods 0-t1 and t3-t4 in which the static focus voltage Vfs is lower than the dynamic focus voltage Vfd, a first dynamic quadrupole lens, acting as a focusing lens (QL1V of FIG. 5B) in the vertical direction and as a diverging lens (QL1H of FIG. 5B) in the horizontal direction, is formed between the third and fourth focus electrodes F3 and F4. A second dynamic quadrupole lens, acting as a diverging lens (QL2V of FIG. 5B) in the vertical direction and a focusing lens (QL2H of FIG. 5B) in the horizontal direction, is formed between the fifth and sixth focus electrodes F5 and F6. After passing through the second dynamic quadrupole lens, the electron beams pass through a main lens ML between the sixth focus electrode F6 and the final acceleration electrode A. Then, electron beams having ovalshapes corresponding to the vertical and horizontal deflecting voltages are output from the main lens ML.
In the period t1-t3 in which the static focus voltage Vfs is higher than the dynamic focus voltage Vfd, a first dynamic quadrupole lens acting as a diverging lens (QL1V of FIG. 5A) in the vertical direction and as a focusing lens (QL1H of FIG. 5A) in the horizontal direction is formed between the third and fourth focus electrodes F3 and F4. Also, a second dynamic quadrupole lens acting as a focusing lens (QL2V of FIG. 5A) in the vertical direction and a diverging lens (QL2H of FIG. 5A) in the horizontal direction is formed between the fifth and sixth focus electrodes F5 and F6. After passing through the second dynamic quadrupole lens, the electron beams pass through a main lens ML between the sixth focus electrode F6 and the final acceleration electrode A. Therefore, electron beams have oval shapes corresponding to the vertical and horizontal deflecting voltages are output from the main lens ML.
In the electron gun for a CRT operating as described, if the CRT has a large screen, the deflecting frequency needs to be increased. Also, to increase the maximum brightness of the CRT, the range of the voltage change of the data signal applied to the electron gun should be increased. However, as the range of a voltage change of the data signal applied to the electron gun increases, the quality of the image deteriorates due to distortion of the data signal.
Accordingly, a method of efficiently driving an electron gun producing increased current density electron beams without increasing the range of a voltage change of the data signal applied to the electron gun is needed.
Referring to Japanese Unexamined Patent Application Publication No. 11-224,618, an additional modulation electrode is provided between a second grid electrode (a screen electrode) and a third grid electrode (a focus electrode). Since a voltage having a negative polarity is applied to the modulation electrode, electron beams having a low current density are cut off and electron beams having a high density current can pass through the modulation electrode. That is, the cathode current can be increased.
However, in the conventional CRT, a leakage current flows through the second grid electrode (the screen electrode) to which a voltage having a positive polarity is applied and between the first grid (the control electrode) and the modulation electrode, so that the life span of the electron gun is reduced.
To solve the above-described problems, it is an object of the present invention to provide a CRT which can efficiently increase cathode current density without increasing the range over which the voltage of a data signal applied to the electron gun changes.
To achieve the above object, there is provided a CRT having an electron gun including, arranged in series, a cathode for an emitting electron beam, a control electrode for controlling emission of the electron beam from the cathode, and a screen electrode for accelerating the electron beam passing through the control electrode, wherein, during a scanning period, a voltage applied to at least one of the control electrode and the screen electrode changes in response to voltage of a data signal applied to the cathode.
In this CRT, the cathode includes a cathode for emitting an electron beam for producing red light, a cathode for emitting an electron beam for producing green light, and a cathode for emitting an electron beam for producing blue light, and the control electrode is divided into a control electrode for red light, a control electrode for green light, and a control electrode for blue light, the control electrodes for red light, for green light, and for blue light being mutually electrically insulated from each other. Further, a voltage is applied to the control electrode for red light during the scanning period changes in response to voltage of a data signal applied to the cathode for producing red light, a voltage is applied to the control electrode for green light during the scanning period changes in response to voltage of a data signal applied to the cathode for producing green light, and a voltage is applied to the control electrode for blue light during the scanning period changes in response to voltage of a data signal applied to the cathode for producing blue light.
Yet another CRT according to the invention includes a cathode for emitting electron beams, a control electrode for controlling emission of the electron beams from the cathode, and a screen electrode for accelerating the electron beams passing through the control electrode arranged in series, wherein, the cathode includes a cathode for emitting an electron beam for producing red light, a cathode for emitting an electron beam for producing green light, and a cathode for emitting an electron beam for producing blue light, and the control electrode is divided into a control electrode for red light, a control electrode for green light, and a control electrode for blue light, the control electrodes for red light, for green light, and for blue light being mutually electrically insulated from each other. In this CRT, the control electrode for red light includes a first beam passing aperture for passing both of the electron beams from the cathodes for producing green light and blue light and a second beam passing aperture for passing the electron beam from the cathode for producing red light and the first beam passing aperture is larger than the second beam passing aperture, the control electrode for green light includes a first beam passing aperture for passing both of the electron beams from the cathodes for producing red light and blue light and a second beam passing aperture for passing the electron beam from the cathode for producing green light and the first beam passing aperture is larger than the second beam passing aperture, and the control electrode for blue light includes a first beam passing aperture for passing both of the electron beams from the cathodes for producing red light and green light and a second beam passing aperture for passing the electron beam from the cathode for producing blue light and the first beam passing aperture is larger than the second beam passing aperture.
A still further CRT according to the invention includes a cathode for emitting electron beams, a screen electrode for screening emission of the electron beams from the cathode, and a screen electrode for accelerating the electron beams passing through the screen electrode arranged in series, wherein, the cathode includes a cathode for emitting an electron beam for producing red light, a cathode for emitting an electron beam for producing green light, and a cathode for emitting an electron beam for producing blue light, and the screen electrode is divided into a screen electrode for red light, a screen electrode for green light, and a screen electrode for blue light, the screen electrodes for red light, for green light, and for blue light being mutually electrically insulated from each other. In this CRT, the screen electrode for red light includes a first beam passing aperture for passing both of the electron beams from the cathodes for producing green light and blue light and a second beam passing aperture for passing the electron beam from the cathode for producing red light and the first beam passing aperture is larger than the second beam passing aperture, the screen electrode for green light includes a first beam passing aperture for passing both of the electron beams from the cathodes for producing red light and blue light and a second beam passing aperture for passing the electron beam from the cathode for producing green light and the first beam passing aperture is larger than the second beam passing aperture, and the screen electrode for blue light includes a first beam passing aperture for passing both of the electron beams from the cathodes for producing red light and green light and a second beam passing aperture for passing the electron beam from the cathode for producing blue light and the first beam passing aperture is larger than the second beam passing aperture.